The known heater comprises a sleeve of thermally conductive silicone rubber in which two spaced guide channels are provided which extend into the rear part of the sleeve and each of which has a contact plate. A PTC heating element is clamped in the front area of the sleeve between the two contact plates which is supplied with electrical current via the two contact plates. The sleeve is closed by a holding body which simultaneously serves as a lead cleat for the feed lines leading to the contact plates.
This heater is designed as a heating cartridge and should be inserted in the device to be heated. Since the contact plates and the PTC heating element are completely surrounded by the sleeve and holding body the heat is transferred from the PTC heating element to the outside longitudinally and transversely to this sleeve so that the thermal yield from the PTC heating element to the object to be heated is determined by the thermal conduction in the silicone rubber and is thus unsatisfactory, which must be seen as a disadvantage.
A further disadvantage is the fact that the sleeve itself must be geometrically adapted to the dimensions of the PTC heating element used so that the resilient clamping of the PTC heating element between the smooth contact plates in the guide channels is sufficiently safe to ensure an adequate heat transfer from the PTC heating element to the contact plates. If the geometric dimensions of the PTC heating element change on account of manufacturing tolerances or special requirements on its output, a completely new sleeve has to be manufactured.
Immersion heaters with PTC heating elements clamped between heat conducting plates through which the electrical power is supplied are known from DE-A 31 36 094 and DE-B-26 14 433. The heat conducting plates of the PTC heating elements are inserted into a glass tube in such a way that the heat conducting plates carry voltage and thus make a good contact with the glass tube on the one hand and to the PTC heating element on the other.
On account of the limited temperatures which can be achieved with PTC heating elements on the one hand and the bad thermal conduction of glass on the other the external thermal yield with these known heaters is also unsatisfactory.
A heater with PTC heating element is also known from DD-C-257 534 in which the PTC heating element is clamped between two serpentine-shaped metal electrodes which serve both to supply the electrical current and carry off the heat.
In today's conventional electrical heaters, which in the widest sense of the word also include fan heaters, heat retaining plates, waffle irons, immersion heaters, heaters for foot baths, aquarium heaters, etc., heating spirals, spiral-wound filaments or other metallic resistance heaters are normally used as heating elements. The temperature yield of the heating elements is generally many hundreds of degrees C., spiral-wound filaments, for example, heat up to over 600.degree. C. and more. On account of the increasing safety requirements a number of regulations now have to be observed by such heaters relating to both the electrical insulation to protect against electric shocks as well as the thermal insulation to protect against burns.
The heat is emitted either by radiation, whereby passing air is heated, or through thermal conduction, whereby the heating element is in contact with a thermally conductive surface, such as is the case with waffle irons.
The aquarium heaters mentioned, for example, often display an immersion body in the form of a glass rod with internal spiral-wound filament which can be inserted into the aquarium's water, whereby there is an air gap between the spiral-wound filament and the glass wall for safety reasons. On account of the bad heat transmission via this air gap and the bad thermal conduction of the glass the spiral-wound filament must be heated up to a very high temperature to ensure an adequate emission of heat to the water. A control loop with at least one probe is generally required for an exact temperature adjustment. Due to the necessarily great difference in temperature between the water and the spiral-wound filament on the one hand and the inertia of the overall system on the other its control is complicated and often has to be re-adjusted by hand. This is particularly undesirable since the adjustment and control mechanism is often located at the top of the glass rod and is immersed with this in the water so that the operator must reach into the water, whereby the fishes are often unwantedly disturbed.
These voluminous immersion heaters also often disturb the appearance of lovingly attended aquariums, though they do have the advantage that they can be retrofitted or replaced without having to change the landscape.
Other aquarium heaters are in the shape of a mat which function in the same way as an electric blanket and are placed on the floor/under the sand in the aquarium. Although they are supplied with 220 volts directly from the mains and are easier to control than the aforementioned glass rod heaters they also display a series of specific disadvantages.
The floor heater is firstly very expensive to construct and secondly requires a large surface area. Its replacement or retrofitting is very complicated, the fishes and the water firstly have to be emptied and the landscape removed to enable free access to the base of the aquarium.
On the whole, spiral-wound filaments also tend to burn out, e.g. as the result of high currents at make or mechanical vibrations when they are still hot, so that frequently the complete heater has to be replaced.
In view of this, an object of the present invention is to create a heater of the type mentioned at the outset which can be manufactured at low cost and is of a simple design whereby this should be such that the heater can be used for a number of applications and displays a good external thermal conduction.